FR2490430A1 - DEVICE FOR CORRECTING AMPLITUDE DISTORTIONS OF RADIO SIGNALS AND RECEIVER COMPRISING SUCH A DEVICE - Google Patents

Abstract

THE INVENTION RELATES TO CORRECTION OF LINEAR AMPLITUDE DISTORTIONS OF RADIOELECTRIC SIGNALS. ITS SUBJECT TO A CORRECTION DEVICE, PLACED IN THE INTERMEDIATE FREQUENCY STAGES OF THE RECEIVER, WHICH INCLUDES A CORRECTION CIRCUIT 1 WHOSE STRUCTURE IS THAT OF A THREE STAGE TRANSVERSE FILTER INTRODUCING THE DELAYS INPUT SIGNAL, O, T2 AND T AND ATTENUATIONS A, A AND A, OR A IS VARIABLE, THIS CORRECTING CIRCUIT ACHIEVING A QUASI LINEAR AMPLITUDE-FREQUENCY CHARACTERISTIC AND A FIXED DELAY T2, WITHOUT VARIATION OF PHASE AS A FUNCTION OF FREQUENCY. THE CONTROL SIGNAL C APPLIED TO THIS CORRECTOR IS OBTAINED BY THE INTERMEDIATE OF TWO FILTERS 7 AND 8 WITH NARROW BANDS SYMMETRICAL WITH RESPECT TO THE INTERMEDIATE FREQUENCY F, FOLLOWED BY DETECTORS 71 AND 81 COUPLES AT THE INPUTS OF A COMPARATOR 9 PROVIDING THE SIGNAL OF CORRECTION COMMAND. THE INVENTION IS APPLICABLE, IN PARTICULAR, TO THE CORRECTION OF THE SELECTIVE FREQUENCY WEAKNESSES OF THE DIGITAL SIGNALS RECEIVED BY THE RADIOELECTRIC RECEIVERS.

Description

The invention relates to a device for correcting

  linear distortion of the amplitude of radio signals, in particular attenuations

  selective radio frequency

  with such a device. The radio links are subject to

  linear distortions and in particular to weakening

  frequency-variable propagation sequences, which are necessary to be corrected by an adaptive circuit so that the amplitude-frequency curve of the demodulated signal is suitable,

  the signal amplitude distortions being thus minimal

  bet. This defect is clearly perceived in the trans-

  radio missions of digital signals.

  There are several types of

  applicable to digital signal transmissions

  HAZARD. For a first type of device, the correction is carried out by correcting the demodulated signal itself, either by an autoadaptive circuit taking into account, for the evaluation of the value of a bit, the amplitude of the transmitted signal with respect to a threshold amplitude function of the previous states and updated at each

  moment, either by a digital signal processing de-

  The corresponding circuits are complex and therefore lead to a significant cost increase for the radio receivers comprising such correction devices. For a second type of device, the correction

  is obtained by a signal processing at frequency

  intermediate, before demodulation0 For this type of

  it is accepted that the spectrum of the signal is symmetrical

  compared to the center frequency before transmission (which is especially true for digital signals due to 'scrambling') and amplitude detection in two narrow bands

  and other of the central frequency allows, in approxi-

  controlling the defect by a linear law to correct it by a variable corrective circuit whose amplitude-frequency characteristic, linear, has a variable slope

  reverse of that which characterizes the fault detected.

  Such a treatment with a linear frequency amplitude characteristic correction circuit introduced,

  at the same time as the desired amplitude variations

  phase variations that are related to the first

  Mieres. However, if the transmission channel introduces, in addition to amplitude distortions, phase distortions, these two types of distortion are not linked by a deterministic relationship. If, at the same time as the law of amplitude correction, a law of variation

  related to it, also affects the

  general, it is possible that the result of this

  tion; at the same time as a correction of the

  amplitude-frequency, a degradation of the

phase-frequency characteristic.

  The object of the invention is a correction device of the second type, that is to say realizing

  an intermediate frequency correction for

  radio signals whose spectrum is normally symmetrical with respect to the center frequency, when

  of the program, which does not have the disadvantage

  mentioned above, that is to say, which does not change the

phase of the signal.

  According to the invention, a device for correcting radio signal amplitude distortions

  with an intermediate frequency input

  at the input of at least one correction circuit, this circuit further comprising an output coupled to the intermediate frequency output of the device by an amplifier and a correction control input, is mainly characterized in that the correction circuit has a three-stage transverse filter structure and comprises for this purpose three channels whose inputs are connected to the input of the introducing circuit

  respectively a delay T and a variable attenuation

  for the first, a delay T and an attenuation

  fixed for the second, and a variable a-

  for the third, T being a parameter dependent on the intermediate frequency f, the signals coming from

  these three ways being added together to deliver the

  corrected at the exit of the circuit.

  The invention will be better understood and other

  features will appear using the description

  which follows with reference to the appended figures.

  FIG. 1 represents a block diagram of one embodiment of the variable correction circuit,

  used in the correction device according to the in-

  vention. Figure 2 shows the transfer function

  of this correction circuit for several values of the

variable variable.

  Figure 3 shows the block diagram of a

  preferred embodiment of the corrective circuit

  raible used in the correction device according to the invention. Figure 4 is a detailed diagram of a circuit

corrector.

  FIG. 5 is a block diagram of the device

  tif correction according to the invention.

  Figure 6 shows curves illustrating the

defect to correct.

  As indicated above, the correcting circuit

  must have, in the frequency band, a characteristic

  substantially linear amplitude-frequency coefficient, with a variable slope and must not change the phase of the signal as a function of the frequency in this frequency band. For this, the correction circuit is of the type

  transverse filter with three stages. The synoptic pattern

  that of such a filter is shown in FIG.

  has a signal input EU two circuits to re-

  late pure, in series 10 and 11 and three attenuators 12, 13 and 14 whose inputs are respectively connected to the input E, the common point I to the two delay circuits 10 and 11, and the output of the second delay circuit 11. The outputs of these three attenuators are connected to the inputs of an adder 15 whose output

  is the outputs of the correction circuit. -

  Both delay circuits introduce

  fixed spots equal to T on the signals they receive.

  The attenuation coefficients of the attenuators 12 and 14 are equal to a and variables simultaneously in the same direction via a control signal applied to their control inputs; the attenuation coefficient of the attenuator 13 is fixed and equal to ao; a signal at the frequency f, amplitude A at the input of this circuit, is transformed into a signal at the same frequency not out of phase and only delayed by T

2 amplitude (2acosufT + ao) A.

  The response of such a circuit as a function of frequency, A (f), is shown in FIG. 2. It is sinusoidal, around a mean amplitude ao A, the

  zeros of this sinusoidal variation being at

  quences f such that: f = 2k-I-, k positive integer or 2Tt zero, and there is no phase variation of the signal

according to the frequency.

  If the band B of the transmitted signal is less than and if the duration T is chosen according to the 2k + 1 central frequency fo of the band such that T - 2f 'o for example with k = 0, f = - then has the answer o 2T e 'of the corrective circuit is comparable to a linear response of slope 2a2T. If a is variable by adjustment between - a1 and + a1 & the slope of the frequency response of the correction circuit varies in - 2a1zT and + 2a1% T, the amplitude at the center frequency fo being

  constant and equal to a oA. Frequency amplitudes-

B B

  fo - 2 and f + 2 are respectively equal to (a + 2a sin X B) A and (a - 2a sin B - T) A, and their o 2 o0 2 2a. BT ratio is equal to 1 + ao sin a 2a BT 1- a sin 2 For example, for B = 61 (if fo = 14OMHz and B = 47 MHz) and for a = 2ao0, the overall slope between frequencies extremes of the frequency band is

of the order of 10 dB.

  Such a correction circuit can thus compensate for a first-order (linear) amplitude distortion of the amplitudefrequency transfer function without

alter the phase.

  In practice, the conventional structure of the three-stage transverse filter shown in FIG. 1 is not perfectly adapted because it is difficult to

  guarantee, by two different attenuators, coeffi-

  mitigation factors exactly equal in the variations

  performed for the correction.

  FIG. 3 represents the block diagram and FIG. 4 is a detailed diagram of an embodiment.

  of the correcting circuit used in the

  tif correction according to the invention. In these figures the same elements as in FIG. 1 have been designated by the same references. In this circuit, the undelayed signal present on the input E and the delayed signal of T are summed before variable attenuation, a single attenuator attenuator attenuating between -a and + a1 being then necessary. The circuit comprises from the signal input E, the two pure delay circuits 10 and 11 in series. As in the first embodiment, the common point I to the two delay circuits is connected to the input of the fixed attenuator 13 of attenuation coefficient a - But the input E and o the output of the circuit to delay 11 are connected to a

  additional adder I6. whose exit is

  linked to the input of a single variable attenuator 17 which then has the same function as the attenuators 12 and 14 of the first circuit. The outputs of the attenuators 13 and 17 are then connected to the output adder,

  15 whose output S is the output of the correction circuit.

  In the two embodiments of the correction circuit described with reference to FIGS. 1 and 3,

  a variant is to use instead of the two cir-

  2 delayed delay circuits 2, whose inputs are connected to the 3rd input

  the one of delay 2 whose output is related to the attenuation

  station 13, the other of delay T whose output is connected to the variable attenuator 14 in the case of FIG. 1, or to the adder 16 in the case of the

figure 2.

  FIG. 4 represents a little more in detail the preferred embodiment of the correction circuit with a single variable attenuator, in the second variant, comprising a delay circuit T and a delay circuit T-2 '. The input is connected to a access of a coupler 3dB, used as a separator. The two access ports of this coupler are respectively connected to the input of a flexible coaxial line 21 of length 1 and to an access of a second coupler, 3dB, 22, also used as a separator. The output ports of this second coupler are respectively connected to the input of a second

  flexible coaxial line, 23, of length 12 and in the

  a resistive compensation attenuator, 24, whose attenuation is adjusted to be equal to the attenuation (low) introduced by the coaxial line

23 (of the order of 0.2 dB to 0.5 dB).

  The outputs of the line 23 and the attenuator 24 are connected to the two inputs of a coupler 3dB,

  25, mounted in an adder whose output sum is

  linked to the input of a PIN diode ring modulator,

  26, used in variable attenuator, the signal of com-

  variable command C acting on the bias current

diodes of the modulator.

  The output of this modulator is connected to an access of another coupler 3dB, 27, mounted in adder,

  the other access of this coupler 27 being coupled to the output

  portion of the coaxial line 21 via a resistance attenuator, 28, for introducing in the delayed channel of T, the fixed attenuation a. and an additional attenuation intended to equal that introduced by the couplers and the coaxial line into

  the channel having the variable attenuator.

  The use in this circuit of a ring modulator as variable attenuator is interesting

  because it is possible to carry out a di-

  by a positive or negative error signal, the attenuation coefficient to vary the slope

  corrector between positive values and values

  their negative ones. The use in this modulator of

  PIN diodes can prevent the generation of harmonics.

  Such a correction circuit can therefore be inserted simply in the intermediate frequency stages of a receiving station, as shown in the diagram of FIG. 5 which represents the embodiment of the present invention.

  simpler of the correction device according to the in-

  vention. In the correction device represented in this figure -the correction circuit 1, made according to one of the embodiments described above, has its

  input E coupled to the frequency signal input

  EFI intermediary via an amplifier

  variable gain IF receiver 2, comprising a

CG gain control.

  Output S of the correction circuit is connected to

  the input of an amplifier 3 compensating by its am-

  plification, the attenuation introduced at the frequency

  central f0 by the correction circuit 1.

  This fixed attenuation a. is a function of the

  the maximum desired correction in the signal band, the signal band, the losses introduced by the couplers and the minimum attenuation of the variable attenuator which is added to the fixed attenuation. The adjustment of the amplification coefficient by

  the amplifier 3 is made permanently. The spell

  part of this amplifier is the output of the floor to

SFI intermediate frequency.

  This SPI output is coupled to the COM input

  automatic gain control, CG of the amplifier 2 d by a band filter, 4, signal in FI, connected

  to a detector, the fifth whose output is connected to the first

  first input of a comparator, 6v the second input of this comparator receiving a reference signal R setting the average level of the signal at the output SFI by command

the gain of the amplifier 2.

  Moreover, for the control of the cor-

  In rector 1, the device comprises two very selective band pass filters having a band of a few MHz, 7 and 8, the central frequencies of which are symmetrical with respect to the central frequency f 0 of the intermediate frequency band of the signal received. Two detectors 71 and 81, which detect the signal levels in these two narrow bands symmetrical with respect to f r have o

  their outputs connected to the inputs of a comparator 9. -

  The output of this comparator which provides a signal of

  command function of the difference of these levels de-

  tected, is connected to the control input C of the corrector circuit 1. The structure described is a feedback control by an error signal indicating the residual slope positive or negative, to be added for

  that the correction is complete. It is also possible

  to carry out the order a priori by connecting the

  filters 7 and 8 no longer at the output SF; but at the

  of the automatic control amplifier 2 of

gain.

  Instead of using a single correction circuit

  to cover the entire correction range (eg

  ple, if the maximum slope to be corrected is 12dB between - B / 2 and + B / 2, a correction circuit whose slope could vary from - 12dB to + 12dB in this band>,

  it is possible to combine several circuits

  similar to that described, these circuits being

  have equal or different T parameters.

  For example, it is possible to associate two correcting circuits, one put into service when the slope to be given to the corrector is positive (between 0 and + 12dB), the other put into service when the slope has

  give the corrector is negative between (- 12dB and 0).

  It is also possible to associate several cor-

  rectors in series, each introducing a slope correction lower than that which would be required by a single corrector making all the correction, for example a first variable slope circuit between 0 and 6dB and a second circuit also having a slope between 0 and 6dB , the two circuits having the same parameter T and being always adjusted in the same way to introduce each half of the

correction required.

  Other combinations may be used, without departing from the scope of the invention to increase the

  dynamic or the linearity of the laws of variation ampli-

  Frequency frequency achievable, for example by combining several corrective circuits having different T parameters. FIG. 6 represents a possible evolution in time of the transfer function T (f) of the radio link: for a band B signal around f0, at the instant t = t1 the slope to be corrected

  is positive, at the instant t = t2 there is a fainting

  deep and at t t3 the slope to correct has become positive. This type of function corresponds to a two-path propagation model. The width of

  lobes, Af = t a of this transfer function is

  payment proportional to the delay T existing between these two paths. The corrector used to correct

  this defect introduced at times t and t respectively

  Negative and positive slope corrections as a function of frequency. On the other hand, the situation represented for t = t2 is not correctable because no

  comparable to an almost linear phenomenon.

  Such a correction system, however, reduces in a ratio t to 5 the duration during which the receiver is not able to evaluate the transmitted signal. It is known that to further reduce this duration of holes due to distortions, it is possible to use a receiver operating in diversity (space or frequency). Indeed, if the same signal is received either by a single antenna but by two antennas in

  a system operating in space diversity, the

  transfer characteristic T '(f) for the second antenna

  will probably be different from that of the first

  at a time t0, therefore, in a receiver

  combining a diversity system with the cor-

  rector according to the invention, the duration of the holes is very small because the correction ranges are large, the probability that the signals received by the two antennas simultaneously have a maximum loss

becomes very weak.

  The invention is not limited to the embodiments

  precisely described and represented. In particular

  In correction devices having, in combination, several correction circuits as described above, it is possible to use other types of variable attenuators than PIN diode ring modulators, especially if each circuit corrector is intended to introduce only variations

positive (or negative) slopes.

  Moreover, it was stated above that in choosing

  setting the parameter T according to the frequency

2k + 1

  is equal to 2f0, the characteristic of trans-

  The correcting circuit is almost linear and thus corrects linear distortions. It is

  possible by appropriately choosing the

  sets T according to the intermediate frequency

  to choose another portion of the sinusoid

  2 in order to correct amplitude distortions showing a curvature, in a given direction, of the transfer function T (f) of FIG.

link.

  Finally, if the need for such a device

  is more clearly perceived for transmissions

  of digital signals, the same

  This can be used to correct amplitude distortions for logic transmissions. Filters that detect the overall slope introduced in

  the signal band by the transmission channel have -

  bands which then coincide with the first pair of symmetrical lateral bands of the signal modulated by the frequency of the highest pilot wave of the signal. in baseband so as to have a signal of value

  constant regardless of the load of the link.

Claims (10)

  1. Device for correcting radio signal amplitude distortions comprising a   intermediate frequency input (E FI) coupled to the   trea (E) of at least one corrector circuit (1), this cir-   further comprising an output (S) coupled to the intermediate frequency output (IF) of the device by an amplifier (3) and a control input of   correction (C), characterized in that the circuit   rector has a three-stage transversal filter structure and for this purpose has three   inputs are connected to the input (E) of the circuit   respectively giving a delay T and an attenuation   a variable for the first, a delay 2 and an atten-   a0 for the second, and an attenuation   variable for the third, T being a functional parameter   intermediate frequency, f, the signals   from these three routes being added together to delimit   set the corrected signal to the output (S) of the circuit.   2. Device according to claim 1, characterized   in that the first and the third way   carry a common variable attenuator (17), the input of the third channel and the delayed input of T of the first channel being connected to a summator <16) whose output is connected to the input of the variable attenuator common (17), the output of this attenuator being connected to an input of an output summator (15)   the other input receives the delayed signal of 2 and   by a fixed attenuator (13) of the second channel.   3. Device according to one of the claims   preceding, characterized in that, in order to achieve the   coefficient variable attenuation a, the circuit comprises   at least one variable attenuator consisting of a   Diode ring LED PINe the correction control input (C) of the correction circuit acting on the   bias current of the modulator diodes.   4. Device according to any one of the   previous indications, characterized in that for the   correction of linear distortions, the parameter T fi-   xing the delays introduced in the channels of the correction circuit is equal to 2k + 1 where k is a positive integer or none. o.   5. Device according to any one of the   preceding statements, characterized in that the first   the first and second channels comprise, for introducing the delays, determined lengths of coaxial line and, at the inputs or outputs of the channels for separating and adding signals, couplers at 3dB.   6. Device according to any one of the   preceding statements, characterized in that several correcting circuits are associated to adjust the   overall characteristic of the correction to the   transfer of the link in the band of frequency transmitted.   7. Device according to any one of the   preceding dications, characterized in that it comprises   in addition, to generate the control signal of   two symmetrical narrow-band filters (7, 8)   relative to the center frequency f. of the band of the intermediate frequency signal processing an intermediate frequency signal each followed by a detector (respectively 71, 81)., the outputs of the detectors being connected to the inputs of a comparator (9) whose output, providing a signal characteristic of distortion; is coupled to the correction control input (C).   8. Device according to claim 7, characterized   in that the correction is obtained by a feedback loop, the filters (7 and 8) being coupled - t'5   at the intermediate frequency output (SFi) of the operative part.   9. Device according to claim 7, characterized   in that the correction is direct, the filters (7 and 8) being coupled to the input of the correction circuit.   10. Receiver of radio signals,   characterized in that it includes a device for correcting   distortion according to any of the   previous indications, and an automatic control loop   gain taking the signal at the frequency output.   intermediate frequency of the device and acting on the amplifier (3) at intermediate frequency to maintain constant the average level at the output SFI